HTML defines the structure of your web document and the content therein. CSS declares various styles for the contents provided on the web document.
HTML and CSS are often called markup languages rather than programming languages, because they, at their core, provide markups for documents with very little dynamism.
Check out this codepen: https://codepen.io/Dillion/full/XWjvdMG
This article covers A-Z about the mobile and web app development process and answers your question on how long does it take to develop/build an app.